A fluid flowing past a solid boundary exerts a shear stress on it due to the viscosity of the fluid. Measurement of this shear stress ‘τ’ is of much interest to fluid dynamics research community since knowledge of variation in wall shear stress is often very useful in analyzing a flow field.
The wall shear stress measurement techniques can be segregated into direct and indirect methods (T. J. Hanratty and J. A. Campbell, “Measurement of Wall Shear Stress” in Fluid Mechanics Measurements, 2nd ed., R. J. Goldstein E., Taylor & Francis, USA 1996, pp. 575-648).
Detailed reviews of wall shear stress measurement techniques based on conventional macro scale sensors (T. J. Hanratty and J. A. Campbell, “Measurement of Wall Shear Stress” in Fluid Mechanics Measurements, 2nd ed., R. J. Goldstein E., Taylor & Francis, USA 1996, pp. 575-648; J. H. Haritonidis, “The Measurement of Wall Shear Stress,” in Advances in Fluid Mechanics Measurements, M. Gad-El-Hak Ed., Springer-Verlag, 1989 pp. 229-261; K. G. Winter, “An outline of the techniques available for the measurement of skin friction in turbulent boundary layers,” Prog., Aeronaut. Sci., vol. 18, pp. 1-57, 1977) and MEMS based sensors (L. Löfdahl, and M. Gad-el-Hak, “MEMS based pressure and shear stress sensors for turbulent flows,” Meas. Sci. Technol., vol. 10, pp. 665-686, 1999; L. Löfdahl, and M. Gad-el-Hak, “MEMS applications in turbulence and flow control,” Prog. Aerospace Sci., vol. 35, pp. 101-103, 1999; J. Naughton, and M. Sheplak, “Modern developments in shear-stress measurement,” Prog. Aero. Sci., vol. 38, pp 515-570, 2002; M. Sheplak, L. Cattafesta, and T. Nishida, “MEMS Shear Stress Sensors: Promise and Progress,” AIAA paper 2004-2006, Proc. 24th AIAA Aerodynamic Measurement Technology and Ground Testing Conference, Portland, Oreg., 28 Jun. 1 Jul., 2004) have been published.
Among the indirect methods the development of thermal shear stress sensors has seen significant advancement in recent years (E. Kälvesten. “Pressure and wall shear-stress sensors for turbulence measurements,” PhD thesis, KTH, Sweden, 1996; C. Liu, C-B. Huang, Z. Zhu, F. Jiang, S. Tung, Y-C. Tai and C-M. Ho, “A micromachined flow shear-stress sensor based on thermal transfer principles,” J. Microelectricalmech. Syst., vol. 8, pp. 90-99, 1999; M. Sheplak, V. Chandrasekaran, A. Cain, T. Nishida, and L. Cattafesta, “Characterisation of a micromachined thermal shear-stress sensor,” AIAA J., vol. 40, pp. 1099-1104, 2002; S. Tung, H. Rokadia, and W. J. Li, “A micro shear stress sensor based on laterally aligned carbon nanotubes,” Sensors and Actuators A:Physical, vol. 133, pp. 431-438, 2007; M. Kimura, S. Tung, J. Lew, C-M Ho, F. Jiang and Y-C Tai, “Measurements of wall shear-stress of a turbulent boundary layer using a micro-shear stress imaging chip,” Fluid. Dyn. Res., vol. 24, pp. 329-342, 1999; J-B Huang, Z. Zhu, F. Jiang, Y-C Tai, and C-M Ho, “A microelectro-mechanical-system-based thermal shear-stress sensor with self-frequency compensation,” Meas. Sci. Technol., vol. 10, pp. 687-696, 1999; Y. Xu, Y-C. Tai, A. Huang and COM. Ho, “IC-integrated flexible shear-stress sensor skin,” J. Microelectromech. Syst., vol. 12, pp. 740-747, 2003). The integration of CTA—Constant temperature anemometry (CTA) and constant current anemometry (CCA) circuits with polysilicon based shear stress sensors is described in X-Q. Qang/Wang, Z. Han, F. Jiang, T. Tsao, Q. Lin, Y-C. Tai, V. Koosh, R. Goodman, J. Lew, and C-M Ho, “A fully integrated shear-stress sensor,” Proceedings of Transducers 99, pp. 1074-1077, 1999.
The sensors described above have been developed using various materials as sensing elements (e.g. polysilicon (E. Kälvesten. “Pressure and wall shear-stress sensors for turbulence measurements,” PhD thesis, KTH, Sweden, 1996; C. Liu, C-B. Huang, Z. Zhu, F. Jiang, S. Tung, Y-C. Tai and C-M. Ho, “A micromachined flow shear-stress sensor based on thermal transfer principles,” J. Microelectricalmech. Syst., vol. 8, pp. 90-99, 1999; M. Kimura, S. Tung, J. Lew, C-M Ho, F. Jiang and Y-C Tai, “Measurements of wall shear-stress of a turbulent boundary layer using a micro-shear stress imaging chip,” Fluid. Dyn. Res., vol. 24, pp. 329-34-2, 1999; Y. Xu et al ibid, platinum (M. Sheplak, V. Chandrasekaran, A. Cain, T. Nishida, and L. Cattafesta, “Characterisation of a micromachined thermal-shear stress sensor,” AIAA J., vol. 40, pp. 1099-1104, 2002), laterally aligned carbon nanotubes (CNTs) (S. Tung, H. Rokadia, and W. J. Li, “A micro shear stress sensor based on laterally aligned carbon nanotubes,” Sensors and Actuators A: Physical, vol. 133, pp. 431-438, 2007)). Diverse thermal isolation schemes have also been employed (e.g. polyimide filled KOH-etched trench (E. Kälvesten. “Pressure and wall shear-stress sensors for turbulence measurements,” PhD thesis, KTH, Sweden, 1996), surface micro-machined vacuum cavity under silicon nitride diaphram (C. Liu, C-B. Huang, Z. Zhu, F. Jiang, S. Tung, Y-C. Tai and C-M. Ho, “A micromachined flow shear-stress sensor based on thermal transfer principles,” J. Microelectromech. Syst., vol. 8, pp. 90-99, 1999), and wafer-bonded vacuum cavity under silicon nitride diaphram (M. Sheplak, V. Chandrasekaran, A. Cain, T. Nishida, and L. Cattafesta, “Characterisation of a micromachined thermal shear-stress sensor,” AIAA J., vol. 40, pp. 1099-1104, 2002). However thermal isolation could be improved.
Further background prior art can be found in: US2006/0081064; US2005/0021247; US2003/0199116; U.S. Pat. Nos. 6,071,819; 5,511,428; and 5,291,781.
In this specification we are particularly concerned with hot film shear stress sensors. In this type of sensor a film of material is located at the edge of the fluid flow, typically inset into a wall or other boundary. The film is heated to above the ambient temperature of the fluid, for example by a few tens of degrees centigrade, by passing electrical current through it. The rate of heat loss from a heated hot-film to the air flow is dependent on the velocity profile in the boundary layer and the viscosity of the fluid. The shear stress ‘τ’ is mathematically defined as:τ=μ(dUy/dy)  (1)where ‘μ’ is the fluid viscosity and ‘Uy’ is the flow velocity at a distance ‘y’ from the wall. As a result, the flow shear stress determines the rate of heat transfer from the heated element to the surrounding flow field. The temperature change of the hot film can be measured by monitoring the change in its resistance. The resistance of a hot film is given by the relationship:R=R0(1+α(T−T0))  (2)where ‘R’ and ‘R0’ are sensor electrical resistance values at a higher and ambient (or a reference) temperature respectively, ‘α’ is hot film's temperature coefficient of resistance (TCR) and ‘T’ and ‘T0 ’ are the film's temperature and the ambient (or reference temperature), respectively.
To measure the shear stress, the following relationship (T. J. Hanratty and J. A. Campbell, “Measurement of Wall Shear Stress,” in Fluid Mechanics Measurements, 2nd ed., R. J. Goldstein Ed., Taylor & Francis, USA, 1996, pp. 610-612) is used:(I2R)/(T−T0)=A(ρτ1/n)+B  (3)where ‘I’ is the electrical current through the hot film, ‘R’ is the hot film's electrical resistance, ΔT is the hot film temperature rise from ambient or a reference temperature, ‘ρ’ is fluid density and ‘A’, ‘B’ and ‘n’ are constants that are determined experimentally through a calibration procedure. Effectively the term A (ρτ1/n) signifies the heat loss into fluid through convection and B signifies the heat loss to the substrate through conduction.
Three type of driving anemometry circuits are commonly used for sensor operation. These are known as constant temperature (CT), constant current (CC) and constant voltage (CV) circuits and details of these are well-known to those skilled in the art.
Background prior art can be found in U.S. Pat. Nos. 5,883,310, 6,071,819, 6,953,982, 6,825,539, 6,511,859, 5,243,858, and US 2006/0154401.
It would be desirable to be able to employ a CMOS manufacturing process to fabricate a hot film shear stress sensor. In particular this offers the promise of reduced manufacturing costs, option of circuit integration on the same chip, reproducible sensor geometry and characteristics, mass production, and reliable sensor performance There have been few attempts to design CMOS hot film shear stress sensors in past (ibid, X-Q. Wang et al ibid; and Y. Xu ibid). However, these have employed polysilicon as a sensing material. Compared to metals (e.g. tungsten, aluminium), polysilicon has some significant disadvantages when used as a hot film shear stress sensor: it has a lower TCR, a lower thermal operating range as its resistance starts to change/drift at higher temperatures, higher impedance, significant 1/f noise (S.-L. Jang, “A model of 1/f noise in polysilicon resistors,” Solid-State Electronics, 33, 1155-1162, 1990) and much higher coefficient of piezo-resistivity which causes piezo-resistivity induced resistance changes in the sensor in addition to shear stress induced resistance variations.